The present invention relates in general to sweep generators and in particular to a method and apparatus for producing a set of parameters characterizing gain and offset errors in a sweep generator output.
Oscilloscopes typically employ at least one sweep generator to produce a voltage ramp output signal ("a sweep signal") for controlling the horizontal movement ("the sweep") of an electron beam across the screen of a cathode ray tube. When the vertical movement of the beam is controlled by an oscilloscope input signal generated by a device under test, the resulting waveform display on the screen represents the magnitude of the input signal as a function of time. The oscilloscope sweep generator initially maintains the sweep signal at a constant, predetermined starting level until the sweep is initiated by a trigger signal. Thereafter, the sweep signal voltage rises with a constant slew rate to a predetermined ending level. Then the sweep signal returns to its starting level and remains there until the sweep is retriggered.
The horizontal position of the beam on the screen is proportional to the sweep signal voltage, and the nominal starting and ending levels of the sweep signal may be chosen so that the beam nominally starts and ends on the left and right edges of a grid superimposed on a waveform display area of the oscilloscope screen. The trigger signal initiating the sweep is typically generated when the input signal reaches a predetermined trigger level, so the trigger point on the resulting waveform display can be nominally located at the left edge of the grid. The grid divides the display area into regular vertical and horizontal divisions to enable an operator to gauge, for any point on the waveform, its magnitude and horizontal displacement from the left edge of the grid. The time delay between the trigger point and a subsequent point of interest along a waveform is typically determined by dividing the displacement (in grid divisions) of the point of interest from the left edge of the grid by the nominal "sweep rate" of the beam, a measure of the speed at which the beam travels across the screen expressed in terms of grid divisions per unit time.
However, the accuracy of such time delay determination is affected not only by the accuracy with which the operator can gauge the horizontal displacement of the point on the screen, but also by the accuracy of the sweep signal. When the actual starting level of the sweep signal does not match its nominal value, the start of the sweep does not really line up with the left edge of the grid and the sweep has an "offset error". When the actual sweep rate differs from the nominal sweep rate, the sweep signal has a "sweep rate" error. Either type of error will cause an error in a time delay determination since the determination would assume the actual sweep signal starting level and sweep rate match their nominal values.
In oscilloscopes of the prior art, the usual approach toward reducing offset and sweep rate errors has been to calibrate the sweep generator to ensure the sweep has the correct offset and sweep rate. But accurate adjustment of sweep rate and offset requires the use of laboratory test equipment and may require partial disassembly of the oscilloscope, and therefore cannot be made frequently so as to account for changes in starting level and sweep rate due to aging of oscilloscope components or changes in operating conditions.
What is needed is an easy to use method and apparatus for reducing inaccuracies in time delay determinations due to offset and sweep rate errors in the sweep signal, without requiring calibration adjustment to the sweep generator hardware.